A modern reflectometry technique is usefully described in an article entitled “In-Situ Characterization During MOVPE Growth of III-Nitrides using Reflectometry” by Christoph Kirchner and Matthias Seyboth, working in the Department of Optoelectronics in the University of Ulm.
In this article, the authors describe an in-situ reflectometry technique during low pressure Metal Organic Vapor Phase Epitaxy (MOVPE) growth of GaN using a commercial fiber reflectometer.
Nitride based materials comprise today's fastest developing III-V compound semiconductor (In—Al—G a—N) technology. Excellent optical and electrical properties, a wide and direct bandgap in combination with high thermal, mechanical, and chemical robustness make GaN and its alloys a well suited material system for optoelectronic devices in the UV to visible frequency range (e.g. light emitting diodes (LEDs), laser, photodetectors).
Successful epitaxial growth of such multilayered device structures requires precise control of the growth parameters (temperatures, flows, pressures) to achieve reproducible results. In particular, the heteroepitaxial GaN growth on highly mismatched substrates requires a two-step growth process consisting of                i. nucleation at low temperature to provide a nucleation semiconductor layer and annealing this layer, and then        ii. subsequent semiconductor growth to achieve high quality epitaxial GaN layers.        
Deposition and subsequent annealing of the nucleation layer is a critical, highly sensitive process, and reproducibility is a pervasive problem due to the fact that small variations of substrate temperature and slightly different morphologies of the sapphires of which the substrates are commonly constituted strongly influence properties of the nucleation layer and subsequent GaN growth. In-situ characterization methods would therefore be very helpful in controlling the initial growth stages of GaN as this could result in a more uniform, less flawed and more consistent semi-conductor material.
It is worth mentioning that one real-time semiconductor property characterization method in current use is known as reflection high electron energy deflection (RHEED), and this method is widely used in molecular beam epitaxy (MBE) to control two-dimensional growth, growth rates and composition of ternary layers. However, CVD does not involve high vacuum conditions and therefore RHEED cannot be applied.
In gas phase epitaxy (GPE) processes, or other semiconductor layering, deposition and growth techniques which are conducted in aggressive environments, in-situ reflectometry can provide similar access to the growth process.
The most common methods of growing GaN and like semiconductors is a process known as Gas Phase Epitaxy (GPE) or MOVPE, and such process is most commonly carried out using a piece of apparatus known as a reactor. Such reactors are manufactured by companies like Aixtron, Veeco, and EMF Limited. A specific example of a reactor, and one which is currently popular in the industry is an Aixtron AIX 200 RF. Essentially, the reactor is a horizontally orientated cylindrical chamber through which gas vapour is allowed to flow and which is radio-frequency heated and comprises a water cooled quartz reaction chamber operated at low pressure. Typically, Trimethylgallium (TMGa), Trimethylindium (TMIn), Trimethylaluminum (TMAl) and ammonia are used as group III and group V precursors respectively and these are caused to pass over a substrate material, which is commonly sapphire (Al2O3).
Referring firstly to FIG. 1 provided herewith, the MOVPE system was equipped with a commercially available reflectometer schematically indicated at 2 consisting of a white light source 4 and a CCD spectrometer 6 (Filmetrics F 30). The spectrometer is a 512-element photodiode array with a spectral range of 400 nm-1100 nm and a resolution of 2 nm. The spectrometer is controlled by a computer 8 and the spectrometer software allows calculation of semiconductor physical characteristics such as deposition rate, the refractive index n, the extinction coefficient k and reflectivity. For these purposes, material data libraries are contained in the software.
As will be appreciated from FIG. 1, an optical access to the substrate with the nitride layer growing thereon in the MOVPE reactor is mandatory.
Accordingly, the reactor 10 comprises a liner tube 12 made of quartz glass. To the outside of the reactor, there is provided a water-cooled jacket 14, and to the outside of said jacket there is provided a radio-frequency heating coil 16 which acts to direct high intensity RF energy onto a susceptor 18 on top of which is positioned a substrate 20 which is most commonly made of sapphire. During use, a source of mixed metal organic gases passes into the chamber through an inlet 22 and as a result of the controlled conditions within the reactor and the composition of the inlet gas, semiconductor material begins firstly to nucleate on the substrate, and subsequently grow thereon. A source of purging gas is also provided which flows around the liner tube and whose flow ultimately aids in the expulsion of the metal organic gas stream from the reactor in general. It is to be understood that the nature of the gaseous flows used in such reactors is often exceptionally toxic to humans, and that great care must be taken in how such gases are handled.
In use, due to the horizontal configuration of the reactor, the ceiling of the liner gets coated with Nitride deposits during semiconductor growth, rendering it opaque to at least some extent. Therefore, a 5 mm diameter hole is drilled in the liner ceiling. The liner is located inside a quartz cylinder (outer reactor tube), which is surrounded by the water cooling jacket made of quartz, too. The reflectometer is mounted directly above the zenith of the usually cylindrical liner in which the hole is drilled so that, except for variations in the surface profile of the semiconductor, light incident thereon from the reflectometer is reflected directly back towards the source of the light as generally indicated at 26. Both the incident and reflected light has to pass through all the quartz walls and the cooling water. Disturbing reflections from the quartz walls can be eliminated by reference measurements as in generally the oscillatory characteristics of the quartz is not affected by reaction conditions.
The spectrometer and the light source are connected to the lens system 28 by optical fibers of a coaxial type, outer strands of which are intended to carry reflected light back to the spectrometer, and the inner strands of which are intended to carry white light from the white light source of the reflectometer. The reflectance of the sample surface, recorded during the growth process, is continuously monitored and recorded. After loading the substrate into the reactor, substrates are typically heated up to 950° C. under a steady flow of a nitrogen/hydrogen mixture. Following this sapphire surface cleaning step, the substrate temperature is lowered to 520° C. for the deposition of the low temperature nucleation layer. After the nucleation layer is deposited, reactor temperature is increased to 1050° C. for growth of undoped bulk GaN.
Reflectance profiles obtained with the above mentioned setup from MOVPE GaN growth processes on sapphire are shown in FIG. 2. The two curves were recorded during GaN growth on sapphire substrates with slightly different polishing delivered from different manufacturers. The deposition of the nucleation layer causes the first increase in reflectivity. During the following annealing step, while the polycrystalline nucleation layer is partially crystallizing, the reflection increases slightly and then drops. At this point the main GaN layer growth is started, revealing small oscillations with increasing amplitude due to decreasing surface roughness. In spite of the fact, that all growth parameters were kept constant, in the initial stages of GaN growth, the course of oscillations amplitudes in the two curves is totally different. While in the upper curve, the maximum amplitude is reached after two oscillations, the lower curve reaches maximum after four oscillations. This confirms, that heteroepitaxial GaN growth processes are very sensitive against every small variation of sapphire substrate properties. Development of the surface morphology is indicated by the course of amplitudes in the reflectance spectrum. After a few oscillation periods, the growth conditions are stabilized. The shown oscillations of the GaN growth correspond to a growth rate of 2 μm/hr. The thickness of the GaN which is grown during one oscillation can be approximately calculated using the following equation:DGaN[nm]=λm/2n where λm is the measuring wavelength of the spectrometer in nm and n is the refractive index of GaN at the measuring wavelength. The oscillations are resonances of the layer system, where the resonator is formed by the GaN layer and the refractive index steps of the transitions GaN/sapphire and GaN/gas phase, respectively. In FIG. 2, one oscillation corresponds to a GaN layer thickness of around 118 nm, according to the above equation. The refractive index of GaN at the spectrometer wavelength of 580 nm is 2.45 and does not change much with temperature. Thus the values for thickness calculated during growth (hot substrate) agree well with data measured at room temperature using Scanning Electron Microscopy (SEM).
During ternary layer growth (InGaN, AlGaN), prereactions in the reactor between the different group III molecules and ammonia can occur, strongly affecting growth rates and composition. The intensity of the prereactions is dependent on pressure and temperature in the reactor during growth and the type and amount of group III molecules (e. g. TMGa, TEGa, TMAl). In-situ reflectometry provides direct information on any change of growth parameters (pressure, temperature, fluxes) affecting either growth rate (change of oscillation width) and/or surface roughness (change of oscillation amplitude).
Other technical articles, specifically one mentioning one of the inventors herefor, namely that published in the Journal of Crystal Growth 248 (2003) 533-536, clearly demonstrate the strong interaction between growth conditions, the substrate surface preparation, and the physical properties of GaN epilayers.
It is also to be noted that other characterisation methods for determining physical properties of semiconductors are available, such as transmission or scanning electron microscopy (T/SEM), high resolution X-ray diffraction (HR-XRD), photoluminescence (PL) and capacitance-voltage (C-V), but such are not suited or indeed impossible to conduct in real-time during the semi-conductor growth process due to the aggressive ambient conditions within the reactor.
There are a number of difficulties associated with the above described in-situ reflectometry technique. Firstly, the coaxial structure of the coaxial fiber optic cable used in the reflectometer, necessitates expensive focussing and light reception optics.
Secondly, the operating temperature of the reactor is commonly in excess of 1000° C., and to ensure that the water does not boil in the cooling jacket, it must be pumped therefore at a sufficient flow rate so that the heat of the reactor can be safely transmitted to the water and thus removed. The difficulty with this is arrangement is that the pumping of water through an essentially annular passageway at a substantial flow rate and pressure necessarily causes some degree of turbulence in the fluid. As a result, the effective refractive index of the fluid through which both the incident and reflected light must pass is slightly altered. It is also to be mentioned that the transfer of heat to the cooling water can also cause some slight change in the refractive index, and therefore any measurements taken from the reflectometer need to take account of this. A useful parallel the these phenomena is the twinkling of stars in a night sky, which is caused by exactly the same dynamic alteration in the refractive indices of space and the earth's atmosphere.
Indeed, the refractive indices of all the various fluids and solids through which the incident and reflected light pass needs to be taken into account in preparing useful data for analysis, and which might ultimately be used to determine the physical characteristics of the semiconductor under test.
It is an object of the following invention to provide an improved means for real-time monitoring of semiconductor characteristics during growth which overcomes the above problems, and provides improved data for analysis.